Neuronal demyelination is a deleterious condition characterized by a reduction of myelin protein in the nervous system. Myelin is a vital component of the central (CNS) and peripheral (PNS) nervous system, which encases the axons of neurons and forms an insulating layer known as the myelin sheath. The presence of the myelin sheath enhances the speed and integrity of nerve signal in form of electric potential propagating down the neural axon. The loss of myelin sheath produces significant impairment in sensory, motor and other types of functioning as nerve signals reach their targets either too slowly, asynchronously (for example, when some axons in a nerve conduct faster than others), intermittently (for example, when conduction is impaired only at high frequencies), or not at all.
The myelin sheath is formed by the plasma membrane, or plasmalemma, of glial cells—oligodendrocytes in the CNS, and Schwann cells in the PNS. During the active phase of myelination, each oligodendrocyte in the CNS must produce as much as approximately 5000 μm2 of myelin surface area per day and approximately 105 myelin protein molecules per minute (Pfeiffer, et al. (1993) Trends Cell Biol. 3: 191-197). Myelinating oligodendrocytes have been identified at demyelinated lesions, indicating that demyelinated axons may be repaired with the newly synthesized myelin.
Neuronal demyelination is manifested in a large number of hereditary and acquired disorders of the CNS and PNS. These disorders include Multiple Sclerosis (MS), Progressive Multifocal Leukoencephalopathy (PML), Encephalomyelitis, Central Pontine Myelolysis (CPM), Anti-MAG Disease, Leukodystrophies: Adrenoleukodystrophy (ALD), Alexander's Disease, Canavan Disease, Krabbe Disease, Metachromatic Leukodystrophy (MLD), Pelizaeus-Merzbacher Disease, Refsum Disease, Cockayne Syndrome, Van der Knapp Syndrome, and Zellweger Syndrome, Guillain-Barre Syndrome (GBS), chronic inflammatory demyelinating polyneuropathy (CIDP), multifocual motor neuropathy (MMN), Alzheimer's disease and progressive supernuclear palsy. For many of these disorders, there are no cures and few effective therapies.
Multiple sclerosis is the most common demyelinating disease of the central nervous system, affecting approximately 1,000,000 people worldwide and some 250,000 to 350,000 people in the United States. The disease is characterized clinically by relapses and remissions, and leading eventually to chronic disability. The earlier phase of multiple sclerosis is characterized by the autoimmune inflammatory strike against myelin sheath leading to paralysis, lack of coordination, sensory disturbances and visual impairment. The subsequent chronic progressive phase of the disease is typically due to active degeneration of the myelin sheath and inadequate remyelination of the demyelinated lesions (Franklin (2002) Nat. Rev. Neurosci. 3: 705-714; Bruck, et at (2003) J. Neurol. Sci. 206: 181-185; Compston, et al. (2002) Lancet 359: 1221-1231).
The precise etiology and pathogenesis of this disease remain unknown. However, pathologic, genetic, and immunologic features have been identified which suggest that the disease involves inflammatory and autoimmune basis. See, for example Waksman, et al. (1984) Proc. Soc. Exp. Biol. Med. 175:282-294; Hafler et al. (1987) Immunol. Rev. 100:307-332. It is now known that pleotropic cytokine interferon-γ (IFN-γ) which is secreted by activated T Lymphocytes and natural killer cells, plays a deleterious role in immune-mediated demyelinating disorders including MS and experimental allergic encephalomyelitis (EAE) (Popko et al. (1997) Mol. Neurobiol. 14:19-35; Popko and Baerwald (1999) Neurochem. Res. 24:331-338; Steinman (2001a) Mult. Scler. 7:275-276). This cytokine is normally absent in the CNS, and becomes detectable during the symptomatic phase of these disorders (Panitch (1992) Drugs 44:946-962). In vitro studies have shown that IFN-γ is capable of promoting apoptosis in purified developing oligodendrocytes (Baerwald and Popko (1998) J. NeuroSci. Res. 52:230-239; Andrews et al. (1998) J. Neurosci. Res. 54:574-583; Feldhaus et al. (2004) J. Soc. Gynecol. Investig. 11:89-96). Despite these extensive studies, the precise mechanism by which the secretion of IFN-γ leads to oligodendroglial abnormalities and alteration to the myelin sheath is not well understood.
There thus remains a considerable need for compositions and methods applicable for elucidating the molecular bases of neuronal demyelination. There also exists a pressing need for developing biologically active agents effective in treating demyelination disorders.